1. Field of the Invention
This invention relates to radio transmission systems in which a plurality of transmitters intermittently transmit short messages indicative of status of sensors associated with the transmitters, to a method of synchronization suitable for using frequency hopping carrier in such systems, and to a method of authentication of the received messages.
2. Description of the Prior Art
In certain types of radio transmitter systems there exist many transmitters that periodically transmit very short messages to one or more receivers. One example of such systems is burglary and fire alarm systems. In these systems, many transmitters located at different places in a building transmit messages indicative of the status of monitoring sensors to a receiver that collects the data from the sensors. Normally, the transmitters transmit supervisory status messages that are as short as feasible and the period between the transmissions is as long as feasible in order to minimize the average current drain as the transmitters in these systems are typically battery operated. In addition, short and infrequent transmissions lower the probability that the data is lost due to collisions which occur when two or more transmitters transmit at the same time. However, when an alarm condition occurs, a transmitter transmits immediately in order to convey the alarm message with little delay.
Typically, such systems transmit data at a single frequency. Thus, they are susceptible to interference and signal loss due to phenomena known as multipath fading. Consequently, the reliability of such systems is compromised or conversely, the transmitted power has to be increased to overcome the fading, which results in a larger power drain and a shorter battery life. Besides, there usually are regulatory limits that restrict such transmitter power and thus limit the possible compensation by sheer increase of power. Since the multipath effect is highly sensitive to the frequency of the transmitted carrier, the frequency hopping spread spectrum technique has a potential to eliminate these drawbacks. However, frequency hopping systems require long acquisition time and they are typically used in two way communication applications in which all the devices are continuously synchronizing with one master device or with each other using a variety of synchronization methods as shown in some of the following references. In other cases, to ease the synchronization problem, there are employed multichannel receivers that can simultaneously receive signals at many frequencies by making the receiver broadband or by using several receivers at the same time. Generally, those solutions suffer from performance degradation or high cost or both, which makes them less desirable for low cost applications that require high reliability such as security alarms.
For example according to U.S. Pat. No. 4,843,638 granted to Walters, a receiver local oscillator has a comb spectrum. This effectively makes it a wide band since each of the frequency range down-converted by the spectral components of the local oscillator will fall in the receiver pass-band and raises the noise floor at the detector input. Consequently, the sensitivity of such receiver will be adversely affected.
In another example according to U.S. Pat. No. 5,428,602 granted to Kemppainen, each hoppping frequency is monitored by a separate receiver. This is very costly and presently not suitable for low cost systems.
In another example, according to U.S. Pat. No. 4,6149,45 granted to Brunius et al., a system is described that allows multiple instruments to be monitored and data to simultaneously transmitted by several radio transponders. However, in order to operate properly, the transponders have to be energized by a RF signal to begin a transmission sequence. This necessitates a radio receiver to be included in the transponder. This makes the system a two-way communications system. Such systems are inherently more complex and costly than one-way communications systems.
In yet another example, according to U.S. Pat. No. 5,659,303 granted to Adair, a transmitting apparatus is described that transmits bursts of data continuously at varying time intervals and at varying frequencies. However, the apparatus as described in the preferred embodiment and associated claims, allows identical hopping pattern to be realized in the transmitters. A means is provided to offset the starting point of the variation sequence for different transmitters depending on the transmitter ID, so that the hopping sequences in various transmitters are initially offset in respect to each other. However, due to unavoidable reference frequency drifts that are different in various transmitters, the sequences may slide in respect to each other. Consequently, it is only a matter of time that the patterns of two or more transmitters will be aligned thus producing a condition for persistent collisions of the transmitted data bursts. In addition, Adair does not provide for a receiving apparatus or a method that would allow such transmitted signals with varying frequencies to be received. In the case of Adair's invention, the actual sequence used by a transmitter is not predetermined but instead it may vary with temperature and depends on the transmitter circuit design and manufacturing tolerances, therefore the signal acquisition is made even more difficult.
A serious problem that must be addressed in battery operated systems is to shorten the transmission time as much as possible by making the message preamble as short as possible in order to conserve the battery power. Therefore, the synchronization of the receiver with the transmitters is a difficult task. This problem is exacerbated in some systems such as security alarms that require some messages to be conveyed to the system immediately without waiting for the scheduled transmission time. A related problem in battery operated systems is a limitation of the transmitted power in order to conserve the battery power. The frequency hopping system, if designed properly, can be advantageously used to combat multipath fading that is a major source of transmitted signal attenuation. Consequently, proper method and construction of the receiver is of great importance. The system design and the receiver design should be done to support each other advantageously.
For example according to U.S. Pat. No. 5,428,637 granted to Oliva, et al., a method is described to reduce the synchronization overhead in frequency hopping systems to reduce the burden of re-synchronization before each separate transmission. The method is based on allocation of specific time slots for any unit that desires to transmit data and thus the method requires a two-way communications to accomplish the necessary exchange of series of reservation and acknowledge messages.
Similarly, in yet another example, according to U.S. Pat. No. 5,438,329 granted to Gastouniotis et al., a two-way system is used for an efficient operation of a telemetry system designed to operate in the presence of multipath fading and interference.
Recently, multicarrier receivers, i.e. receivers that can simultaneously receive symbols at many frequencies using FFT (Fast Fourier Transform), become popular especially in the area of telecommunications. Specifically, such multicarrier receiver created the greatest interest in the area of digital subscriber loop where multiple implementations have been pursued to improve the data rate in the frequency selective communications channels by simultaneous use of multiple modulated carriers. The disadvantage of these implementations of multicarrier transceivers is that fairly complex signals are produced in the transmitters that require expensive and power hungry logic. Thus, these implementations are not suitable for the low cost and low power transmitters used for sensor monitoring.
Another implementation of multicarrier receiver is described in U.S. Pat. No. 5,548,582 granted to Brajal, et al., is aimed at reception of frequency hopping modulated carrier. The designer stated objective was to receive fast hopping modulated carrier using multicarrier receiver principle based on FFT in order to avoid the need to construct fast and phase accurate hopping controlled oscillator in the receiver/transmitter stations.
The system is in principle designed for two-way communication and as such is not suitable for low cost implementations such as required for alarm systems.
Another disadvantage of this method is that it uses a fast frequency hopping in which the data content of the transmitted message determines the sequence of the transmission frequencies and where several frequencies are used to transmit one symbol. Fast frequency hopping requires faster processing in the receiver and the transmitters and results in a more complex and expensive system.
U.S. Pat. References: Pat. No. Inventor Issued Title 4843638 Walters 6/89 "Receiver for frequency hopped signals." 5428602 Kemppainen 6/95 "Frequency-hopping arrange- ment for a radio communication system." 4614945 Brunius et al. 9/86 "Automatic/remote RF instrument reading method and apparatus." 5659303 Adair 8/97 "Method and apparatus for transmitting monitor data." 5428637 Oliva et al. 6/95 "Method for reducing synchro- nizing overhead of frequency hopping communications systems." 5438329 Gastouniotis et al. 8/95 "Duplex bi-directional multi- mode remote instrument reading and telemetry system." 5408506 Mincher et al. 4/95 "Distributed time synchro- nization system and method." 4653068 Kadin 3/87 "Frequency hopping data communication system." 4606041 Kadin 8/86 "Frequency hopping data communication system." 5390166 Rohani et al. 2/95 "Method for recovering a data signal using diversity in a radio frequency, time division multiple access communication system." 5546422 Yokev et al. 8/96 "Method for transmitting low- power frequency hopped spread spectrum data." 5079768 Flammer 1/92 "Method for frequency sharing in frequency hopping communi- cations network." 5548582 Brajal et al 8/96 "Multicarrier frequency hopping commmunications system" 5121407 Partyka et al. 6/92 "Spread Spectrum Communi- cations System."